The calcium-linked hetero-association of human complement subcomponents C1r and C1s has been quantitatively characterized as a function of calcium concentration via the technique of tracer sedimentation equilibrium. The role of macromolecular crowding in the regulation of cellular volume has been further explored. A new technique for the detection of associations in mixtures of several protein components, called sedimentation equilibrium- quantitative polyacrylamide gel electrophoresis (SE-QPAGE), has been developed and tested on mixtures of up to four protein components. Theoretical models have been proposed for the (i) effect of confining proteins to small pores of different shapes, and (ii) and the effect of weak nonspecific attractive interactions between surfaces and proteins upon various conformational and association equilibria.